Hollow metal doors are typically constructed of steel stiffeners or a laminated core sandwiched between relatively thin face sheets of carbon steel. The steel stiffeners are usually open-walled thin sections: channel, Z-shapes, hat-shapes, truss sections, or similar members, positioned vertically and attached to the steel face sheets by spot welding. Laminated cores employ either honeycomb, polyurethane, polyisocyanurate, or polystyrene that are laminated to the face sheets using structural adhesive. Certain self-adhesive polyurethane cores can also be foamed-into the cavity between the face sheets. Hollow metal doors are both strong and lightweight, making them desirable for use in a wide variety of architectural applications as security and fire doors in commercial, residential, industrial, and detention settings.
There are four types of steel commonly used in hollow metal door manufacturing. Cold-rolled steel is the most commonly used material for the majority of commercial hollow metal door components. Hot-rolled steel is used in hardware reinforcements. Zinc-coated steel sheets can be either galvannealed or galvanized and are used to prevent corrosion. Stainless steel is also used for corrosion protection, and for aesthetic purposes.
The current methods for constructing hollow metal doors are detailed in the Hollow Metal Manufacturers' Association (HMMA) standard 802-07 Manufacturing of Hollow Metal Doors and Frames. The four primary steps involved in manufacturing the steel face sheets and stiffeners are shearing, blanking, brake forming, and welding. The hollow metal door fabricator's raw material usually consists of flat steel sheets. These sheets are cut to exact sizes using a shearing machine. The sheared steel then has desired openings blanked, using a punch press, turret press, or a laser cutting machine. The blanked steel is then brake formed into the desired shape using a press brake.
There are now typically at least two steel face sheet portions, to which steel stiffeners are attached by either spot or projection welding. Other components, such as hinge supports and closer reinforcements, are also spot- or projection-welded to the face sheets. This assembly process is cumbersome and time consuming, because numerous components have to be welded into place, and also because parts that do not fit exactly must be modified to fit. Most conventional hollow metal doors that with steel stiffeners spot welded in this fashion require over 600 spot welds in addition to the MIG welding required to assemble the two steel face sheet portions.
Once these two face sheet portions and attached stiffeners, hinge supports, and closer reinforcements are completed, they are then welded together by a continuous weld along the edges. Any hat channels used as stiffeners must also be welded together at this time. End channels are welded into place at each end. All pre-drilled screw holes in these assemblies usually must be re-drilled and re-tapped due to deformation that occurred during bending, welding, and assembly. Finally, the assembled door must be finished and painted before being shipped to the customer. Finishing usually involves a significant amount of grinding and polishing because of the numerous welds on the surface of the door.
A standard commercially-available 3′0″×7′0″ hollow metal door and frame contains at least 70 pieces and takes approximately 8 to 10 hours to produce. The lead time required to produce the component parts can be significant. For example, the lead time required to produce a typical order of 100 hollow metal doors can be on the order of 8-12 weeks. The assembly process for most hollow metal doors is best suited for an elaborate assembly line so that all of the pieces can be added efficiently. Costs to manufacture are therefore higher because of the large amount of parts and labor required to produce these doors.
Moreover, current assembly lines make it almost impossible for most manufacturers to produce a flat door. Usually the door may bow in a variety of directions. This is because conventional doors are manufactured on a series of assembly line roller tables passing through a number of spot welding stations. As spot welders apply pressure to the product, the component parts of the door flex because of the heat and pressure. This often results in permanent warping of the door after the weld is complete. If this warping is significant enough, rework may have to occur before the door is acceptable to the customer.
What is needed is a hollow metal door design that reduces the number of parts and welding required, without sacrificing these doors' light weight and strength.